1. Field of the Invention
The present invention relates to an apparatus and method for converting a DC power.
2. Background of the Related Art
FIG. 1 is a block diagram illustrating a conventional computer system. As shown in FIG. 1, a bus or a data line connects a CPU (Central Processing Unit), a power supply 10, and a LCD (Liquid Crystal Display) of a display unit.
The power supply 10 includes an AC (Alternating Current) adaptor 20 that rectifies an alternating current voltage that is a power source, a battery 30 that is installed in the interior of the system and is used as an assistant power, and a DC/DC converter 40. The DC/DC converter is connected with the above loads and increases and decreases the DC voltage.
A portable device such as a notebook computer, etc. generally includes the DC/DC converter 40 capable of increasing and decreasing the power source for achieving a desired level of power from the AC adaptor 20 or the battery 30. Accordingly, the portable device system uses the power having a certain level.
FIG. 2 is a block diagram illustrating a system that includes a power block adapted in the conventional art that can be adapted according to the present invention. As shown in FIG. 2, there are provided an AC adaptor 20 capable of converting an input AC power into a DC power, and a battery 30 used as an assistant power.
Power is selectively switched by a load switch and is applied to each power block 60. The power is applied to each element (e.g., CPU, etc.) of the system through each power block.
FIG. 2 is a view illustrating a power block of a notebook computer system. Each power block shows different efficiency characteristics. FIG. 3 is a circuit diagram of a buck DC/DC converter 50 having 1.1 volts used by the CPU power with a large variation in a load characteristic in the power block.
As shown in FIG. 3, the buck DC/DC converter 50 includes an input capacitor Ci driven with an input power, voltage Vi or a current source, one or more than one FET 50b, 50c that performs a switching operation from a source input terminal to an output terminal. The DC/DC converter also has an IC 50a that controls the switching unit and an inductor (L) 50d that is adapted for a voltage source.
FIG. 4 is a circuit diagram of an inductor (L) 50d of FIG. 3 where an input terminal 1 and an output terminal 2 are fixed. In the circuit diagrams of FIGS. 3 and 4, the efficiency of the circuits could be computed based on the following equation 1.
                                                                        Eff                ⁡                                  (                  %                  )                                            =                                                P                  OUT                                                  P                  IN                                                                                                        =                                                P                  OUT                                                                      P                    OUT                                    +                                      P                    LOSS                                                                                                                          =                                                                    V                    OUT                                    ⁢                                      I                    OUT                                                                                                              V                      OUT                                        ⁢                                          I                      OUT                                                        +                                      P                    LOSS                                                                                                          [                  Equation          ⁢                                          ⁢          1                ]            
FIG. 5 is a diagram illustrating circuits or circuit components that affect a loss of the above circuit. As shown in FIG. 5, input parameters are formed of circuit devices such as voltage, inductor, FET, capacitance, IC, etc. The inductor can be determined to most affect the loss of the circuit. The inductor can cause over 40% in the total loss. The loss of the inductor may be expressed in the following equation 2.Lloss=Io^2*Rdc—L+(ILrpl/2)^2*Rac—L(where Rdc—L: DCR of Inductor, ILrpl: Inductor ripple current, Rac—L: ACR of Inductor)  [Equation 2]In the equation 2, the loss of the inductor is formed of a loss caused because of a winding resistance and a core loss.
The inductor has DC resistance loss, skin effect loss, hysteresis loss, eddy current loss, etc. In addition, the loss can be classified into a loss caused due to a DC resistance component and a loss caused due to an AC resistance component with respect to an output ripple current based on the cause of the loss.Inductor loss=copper loss+core loss  [Equation 3]
In equation 3, the copper loss represents a loss caused due to a DC resistance component and a resistance component based on a skin effect. The core loss is formed of an eddy current loss and a hysteresis loss.
In the conventional (buck) DC/DC converter, the equation for selecting the inductor can be expressed as follows.
                    L        =                                            (                                                V                  i                                -                                  V                  S                                -                                  V                  O                                            )                        *            D            *                          T              s                                            I            ripple                                              [                  Equation          ⁢                                          ⁢          4                ]            As shown by the equations 2 and 4, the inductance L of the inductor is in inverse proportion to ILrpl, and ILrpl is in square proportion to the loss of inductor.
Accordingly, it is possible to decrease the loss of the inductor by increasing the value L. When the value L is increased, the values Rdc_L and Rac_L are changed. Since the variation of the value Ilrpl is much larger, it is possible to improve the loss of the inductor.
FIG. 6 is a graph of a simulation formed by adapting the loss of each circuit device of FIG. 5 with respect to the conventional converter of FIG. 3. Namely, there is shown an efficiency curve in the entire load sections of the CPU, and DC/DC converter.
As shown in FIG. 6, in the overall or entire load sections, the efficiency of over 80% is achieved. However, the efficiency in FIG. 6 is largely decreased when the load current is low. The efficiency is low in the section 0-2 A (in particular 0-0.2 A) that is an actual load section during the use of the CPU.
Thus, the efficiency is not uniform in the entire load sections 0-13.5 A. It means that the loss is changed by the load section.
A certain or calculatable efficiency difference can be seen when the CPU load is high, and the CPU load is low, respectively. FIGS. 10 and 11 ate diagrams illustrating the degrees of the loss by each circuit device when the CPU load is high or low. FIG. 10 is a diagram illustrating that the loss is 58.4% due to the inductor when the CPU load is low (for example, 0.2 A). FIG. 11 is a diagram illustrating that the loss is 19.3% due to the inductor when the CPU load is high (for example, 13 A).
As shown in FIGS. 10-11, the ratio of the inductor is largest of multiple causes. As the load is getting more decreased, the ratio is getting increased. When the load is 0.2 A, the ratio is over 58.4%.
Therefore, it is possible to increase the efficiency as long as the loss caused due to the inductor is decreased. Thus, there is a long felt need to increase the efficiency of the DC/DC converter. However, it is known that it is impossible to achieve the loss of the inductor with the fixed value L.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.